Inputs

Outputs

This indicates that a transaction was already in
progress. The current transaction is not affected.

Description

By default, Postgres executes
transactions in unchained mode (also
known as “autocommit” in other database systems). In other words,
each user statement is executed in its own transaction and a
commit is implicitly performed at the end of the statement (if
execution was successful, otherwise a rollback is done).
BEGIN initiates a user transaction in
chained mode, i.e. all user statements after BEGIN command will be executed in a single
transaction until an explicit COMMIT, ROLLBACK or execution
abort. Statements in chained mode are executed much faster,
because transaction start/commit requires significant CPU and
disk activity. Execution of multiple statements inside a
transaction is also required for consistency when changing
several related tables.

The default transaction isolation level in Postgres is READ COMMITTED, where queries
inside the transaction see only changes committed before query
execution. So, you have to use SET TRANSACTION
ISOLATION LEVEL SERIALIZABLE just after BEGIN if you need
more rigorous transaction isolation. In SERIALIZABLE mode queries
will see only changes committed before the entire transaction
began (actually, before execution of the first DML statement in a
serializable transaction).

If the transaction is committed, Postgres will ensure either that all updates
are done or else that none of them are done. Transactions have
the standard ACID (atomic,
consistent, isolatable, and durable) property.

Notes

The keyword TRANSACTION is just a cosmetic alternative to
WORK. Neither keyword need be specified.

Refer to the LOCK statement for
further information about locking tables inside a
transaction.